Spark gap devices



May 17, 1966 Filed Jan. 28, 1963 J. W. CALVESBERT ETAL SPARK GAP DEVICES2 Sheets-Shea?I l May 17, 1966 .1. w. cALvl-:sBERT ETAL 3,252,038

SPARK GAP DEVICES Filed Jan. 28. 1963 2 Sheets-Sliema?l 2 United StatesPatent C) 3,252,038 SPARK GAP DEVICES James William Calvesbert andAlfred Alexander Robinson, Stafford, England, assignors to The EnglishElectric Company Limited, Strand, London, England, a British companyFiled Jan. 28, 1963, Ser. No. 254,331 Claims priority, application GreatBritain, Feb. 2, 1962, 4,002/62 4 Claims. (Cl. 313-243) This inventionrelates to protective spark gap devices.

Protective spark gaps made with metal electrodes are standard equipmenton many types of electrical apparatus and are usually satisfactory wherethe gap breakdown voltage is high and the arc drop and the deteriorationof the electrode surfaces are relatively unimportant.

There are many medium and low voltage applications where the faultcurrent may be as high as 20 ka. for which metal electrodes areunsatisfactory because the iirst operation at such currents maysignificantly damage -the electrode surfaces.

Graphite surfaces are much less eroded by high current arcs than aremetal surfaces so that the initial gap setting and hence the breakdownvoltage may be maintained for many discharges. There is the problem,however, of different relative expansions which may give rise tomechanical damage or variable contact resistance.

An object of the present invention is to provide a spark gap devicecapable of employing graphite electrodes.

According to the invention, a protective spark gap device including twonon-metallic electrodes separated by a spark gap has a said non-metallicelectrode urged into electrical contact with an electrode holder of adifferent material at a-substantially constant pressure irrespective ofdifferent relative expansions in the device.

According to a preferred feature of the invention, the non-metallicelectrode is urged into electrical contact with said electrode holder bya taper plug of the same material as the electrode holder, the taperplug being caused to positively engage a correspondingly tapered hole insaid electrode by the action of tensioning means acting simultaneouslyon the taper plug and on said electrode, the arrangement being such thatthe apex of taper of the plug coincides approximately with the centre ofaction of the tensioning means on the electrode holder.

According to another preferred feature of the invention, the tensioningmeans comprises a bolt attached to the electrode holder and having aheadportion engaging the taper plug.

Further preferred features of the invention will appear from thefollowing description with reference tothe accompanying drawings. FIG. 1is a vertical section of one embodiment of protective spark gap deviceaccording to the invention for the protection of series capacitors usedfor reactive -compensation in a power system, FIG. 2 is a sectional planon the line A-A in FIG. 1, and

FIG. 3 is a vertical section of another embodiment of spark gap deviceaccording to the invention.

Referring now to FIGS. 1 and 2, the spark gap device includes twoelectrodes 10 of graphite providing an annular spark gap 11. Three sawcuts 12 are formed radially at the outer end of each electrode 10 andend faces 13 and side faces 14 are in contact with a copper electrodeholder 15. A conical hole 16 is formed centrally in each electrode 10 toprovide a seating for a taper plug 17 formed of copper. The taper plug17 itself has a central hole 18 which is enlarged at the inner endthereof as indicated at 19 to accommodate the head 20 of a bolt 21. Thebolt 21 is secured to a tapped hole 22 in the electrode holder 15 inalignment with the hole 18. It is arranged that the apex of the -conicaltaper of the plug 17 coincides approximately with the centre ofattachment 3,252,038 Patented May 17, 1966 of the bolt 21 in the hole22, that is, the mid-point of the portion of the bolt 21 in threadedattachment with the hole 22.

Each electrode holder 15 abuts a terminal plate 23, the two plates 23being arranged to provide diagonallyopposed line and earth connectionsas indicated at 24 and 25 respectively.

Three insulators 26 are interposed between each terminal plate 23 and anend-plate 27, the insulators being secured to the end-plate 27 by screws28 and to the terminal plate 23 by studs 29. The end-plates 27 arerigidly coupled together by circumferentially disposed rods 30 which arethreaded at the upper and lower ends thereof. Each rod 30 is encompassedby an insulating sheath 31 which rests on a nut 32 co-operating with thethreaded lower end of the rod 30 and is separated by shims 33 from a nut34 co-operating with the upper threaded end of the rod 30, a further nut35 being provided for securing the assembly together. It will be clearthat adjustment of the spark gap 11 to cover a wide range of voltages iseasily produced by addition or removal of the shims 33.

When the bolt 21 is tightened, the taper plug 17 urges the graphiteelectrode 10 into engagement with the electrode holder 15. The effect ofthe very different expansions of copper and graphite is minimised byarranging for the apex of the taper of the plug 17 to approximatelycoincide with the centre of attachment of the bolt 21 (as describedabove) and by forming the taper plug 17 and the electrode holder 15 ofthe same material. This arrangement allows sliding contact between thetaper plug 17 and the graphite electrode 10 without release of theContact pressure exerted by the pre-stress in the bolt 21, over a widetemperature range.

The embodiment of the device described above is suitable for theprotection of electrical apparatus in the 2 to 20 kv. voltage range forfault'currents up to 20 ka.

Referring now to FIG. 3, in which parts similar to those inFIGS. l and 2bear the same reference numerals, the terminal plates 23 are attached tothe end plates 27 by set lscrews 50. The terminal plates 23 areseparated from each other `by four ceramic insulating pillars 51 whichare circumferentially spaced at equal angular intervals. The amount ofthe separation is adjustable by utilizing accurately machined metalshims 52. Screws 53 secure the end plates 27 to the pillars 51. It willbe noted that in this arrangement of spark gap device, the electrodeholders 15 are insulated from each other by the insulating pillars 51instead of by the insulators 26 shown in FIGS. 1 and 2. The dispositionof the parts of this embodiment of the device is in other respectssimilar to that of the previous embodiment, and the device is suitablefor the protection of electrical apparatus in the 2 to 40 kv. voltagerange for fault currents up to 20 ka.

What we claim as our invention and desire to secure by Letters Patentis:

1. A protective :spark gap device having two nonr'netallic electrodes,wherein at least one of said electrodes is urged into contact with anelectrode holder of a different material at a substantially constantpressure irrespective of different relative expansions in the device, bya taper plug of the same material as the electrode holder, the taperplug being caused to positively engage a correspondingly tapered hole in-said electrode by the action of tensioning means acting simultaneouslyon the taper plug and on said electrode, the arrangement being such thatthe apex of taper of the plug coincides approximately with the centre ofactionof the tensioning means on the electrode holder.

2. A protective spark gap device according to claim 1 wherein thetensioning means comprises a bolt attached to the electrode holder vandhaving a head portion engaging the taper plug.

3. A protective spark gap device according to claim 1 wherein said plughas a conical taper, the axis of the core coinciding with the centralaxis`of the electrode holder.

4. A protective spark gap device according to claim 1 wherein theelectrode holders associated with the electrodes are each in electricalcontact with a separate terminal plate which is separated from an endplate by at l least one insulator, the end `plates being maintained apredetermined distance apart by a plurality of rods interconnecting theend plates.

References Cited by the Examiner UNITED- STATES PATENTS 0 GEORGE N.WESTBY, Primary Emminer.

R. JUDD, Assistant Examiner.

1. A PROTECTIVE SPARK GAP DEVICE HAVING TWO NONMETALLIC ELECTRODES,WHEREIN AT LEAST ONE OF SAID ELECTRODES IS URGED INTO CONTACT WITH ANELECTRODE HOLDER OF A DIFFERENT MATERIAL AT A SUBSTANTIALLY CONSTANTPRESSURE IRRESPECTIVE OF DIFFERENT RELATIVE EXPANSIONS IN THE DEVICE, BYA TAPER PLUG OF THE SAME MATERIAL AS THE ELECTRODE HOLDER, THE TAPERPLUG BEING CAUSED TO POSITIVELY ENGAGE A CORRESPONDINGLY TAPERED HOLE INSAID ELECTRODE BY THE ACTION OF TENSIONING MEANS ACTING SIMULTANEOUSLYON THE FUG-01 TAPER PLUG AND ON SAID ELECTRODE, THE ARRANGEMENT BEINGSUCH THAN THE APEX OF TAPER OF THE PLUG COINCIDES APPROXIMATELY WITH THECENTER OF ACTION OF THE TENSIONING MEANS ON THE ELECTRODE HOLDER.